This proposal seeks funding (R21) under the "Neurotechnology Research, Development, and Enhancement" program to develop an electronically reconfigurable microfabricated tetrode array. Under the proposed research, we will design, fabricate, and test a 3D microfabricated electronically reconfigurable tetrode with multiplexing capability for multi-neuronal recording. Each tetrode will consist of four silicon shanks each having eight recording sites. A three transistor analog multiplexer located next to each site allows each one to be individually selected thus reducing the number of leads (to four for an eight channel electrode). This will make possible the eventual creation of electrodes with hundreds of recording sites. We will start with an eight channel electrode as a proof-of-concept. The four shanks are attached to a silicon backend containing the bonding pads through foldable polyimide links. The connecting polyimide links are only partially cured during the fabrication process and their complete curing following the release results in automatic folding into the desired shape. A silicon platform is fabricated to house 16 tetrode assembly resulting in a total of 64 simultaneous recording at any moment. The fabrication process uses a silicon-on-insulator substrate. The active tetrode will be tested in vivo in hippocampal recordings from awake, behaving rats. Hippocampal place cells are well-understood and have been extensively tested with tetrode. This makes them well-suited to testing new technology. Hippocampus has a very well-understood anatomical structure which will allow quantification of the recording location of each site. This makes it an ideal structure with which to test new recording technology.